The invention in this case relates to an aluminum capacitor foil and to a method for producing such a foil. More particularly, the invention relates to an aluminum electrolytic capacitor foil particularly adapted for use in high voltage AC motor start capacitors and to a method for producing such a foil.
For use in high voltage single section AC motor start capacitors it is desired that the capacitor foil employed exhibit the lowest possible power factor combined with a high capacitance. In addition the structure of the foil should be such that the capacitor employing such a foil be capable of continuous duty (with AC voltage applied) for extended periods, (for example, up to 30 minutes or more).
British Patent Specification 761,196 shows the preparation of aluminum capacitor foil in which an etched aluminum foil is guided first through a forming bath in which forming voltage of 200 volts is employed and then through a forming bath in which a forming voltage of 400 volts is employed. Finally the foil is guided through through a compensating bath in which a voltage of 400 to 550 volts is employed. This British Specification is silent about the composition of the bath and also does not suggest that the foil would be useful in high voltage motor start capacitors.
U.S. Pat. No. 2,052,575 shows a method of forming capacitor foils for electrolytic capacitors in which the foil, after cleaning with hot water acidified with a small amount of an acid such as phosphoric or boric acid after which the foil is electroformed in a solution of phosphoric acid. The foil is then washed to remove all traces of electrolyte and is then subjected to DC treatment in hydrolizable film forming electrolyte and then subjecting the foil to AC treatment in the same type of electrolyte, as a result of which a low power loss film is obtained. The resultant electrode is then washed to hydrolyze portions of electrolyte that are retained by the low power loss film and finally the electrode is subjected to a DC surge treatment in a cold film forming electrolyte. This patent is silent about the power factor of the capacitors employing this electrode. In addition there is no suggestion in this patent of the use of the electrode formed therein for electrolytic capacitors employed as motor start capacitors particularly for high voltage use.
German Patent Specification 2,836,878 shows a two step forming process for an aluminum capacitor foil in which the capacitor foil is first formed in an electrolyte comprising citric acid, ammonia and phosphoric acid and then in an electrolyte of this composition but which in addition contains azelaic acid. This German patent teaches nothing about the use of the foil in motor start capacitors nor does it teach anything about the power factor of the foil.
U.S. Pat. No. 4,437,955 shows a method of producing aluminum capacitor foil exhibiting a good combination of capacitance and mechanical strength including the steps of pre-etching the foil in an electrolyte bath containing hydrochloric acid and at least one of phosphoric acid and nitric acid while being subjected to an AC current then, without removing the hydrochloric acid from the foil, etching the foil by immersing the foil in aqueous electrolyte bath containing sodium chloride and either sodium sulfate or boric acid while subjecting the foil to a DC current until the weight of the foil decreases by at least 7 mg/cm.sup.2. Nothing is stated in this patent about the use of such a foil in motor start capacitors and nothing is stated about the power factor of the resultant foil.